1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus that captures Doppler spectrum images of a plurality of sites and a method of generating ultrasonic images thereby.
2. Description of the Related Art
Ultrasonic imaging apparatuses that obtain blood flow information from a diagnostic site by means of an ultrasonic Doppler method are known. When ultrasound is directed toward a certain diagnostic site with a flow such as blood flow in a subject, the received frequency shifts slightly, relative to the transmission frequency due to the Doppler effect. This Doppler shift frequency is proportional to the blood flow velocity. Blood flow information is obtained by analyzing the Doppler shift frequency. For example, a technique has been adopted for observing time changes in the blood flow information by implementing the pulsed wave Doppler method (PWD) or continuous wave Doppler method (CWD).
The ultrasonic imaging apparatus performs fast Fourier Transform (FFT) for obtained Doppler signals. Then, the ultrasonic imaging apparatus displays a spectrum of the results of that frequency analysis with the frequency f (velocity v) on the vertical axis and time t on the horizontal axis. Directed toward that Doppler spectrum image, various items used in diagnosis are measured.
The function of the heart (cardiac function) has been evaluated by utilizing the ultrasonic Doppler method. For example, by measuring the velocity of left ventricular inflow and the velocity of left ventricular outflow, indices such as left ventricular ejection fraction (E/F) and Tei-Index (Total Ejection Isovolume Index) are obtained in order to evaluate the cardiac function.
Conventionally, the electrocardiographic waveform of a subject is acquired in order to identify the time phase when the left ventricular inflow occurs and the time phase when the left ventricular outflow occurs. Then, indices such as the left ventricular ejection fraction (E/F) are derived by separately measuring the velocity of the left ventricular inflow and the velocity of the left ventricular outflow. In other words, the velocity of the left ventricular inflow and the velocity of the left ventricular outflow are obtained from separate heartbeats in order to obtain the indices described above.
When the left ventricular inflow and left ventricular outflow are obtained from separate heartbeats, it is necessary to recalculate indices such as the abovementioned left ventricular ejection fraction (E/F) by combining the index obtained from measuring the left ventricular inflow and that obtained from measuring the left ventricular outflow.
Therefore, the operation is very complicated. In addition, the left ventricular inflow and left ventricular outflow are measured via different heartbeats, so there is a problem in which indices such as the left ventricular ejection fraction (E/F) become instable or the reproducibility of the indices becomes poor when the heart rate varies.
Moreover, even when the average value of the indices acquired from a plurality of heartbeats is obtained, the accuracy thereof may be insufficient.
In addition, there is an alternative method to set range gates (observation points) at a plurality of sites to acquire the blood flow information at a plurality of sites (e.g., Japanese published examined application H3-203706 and U.S. Pat. No. 3,180,958). For example, blood flow information at two sites is acquired by setting the range gates at the two sites and alternately transmitting and receiving ultrasound toward and from respective sites once each. For example, blood flow information at observation point A is acquired by transmitting and receiving the ultrasound toward and from observation point A once, and subsequently, blood flow information at observation point B is acquired by transmitting and receiving ultrasound toward and from observation point B. Later, by alternately transmitting and receiving ultrasound toward and from observation point A and observation point B once each, blood flow information at each observation point is alternately acquired.
However, as in the method according to the prior art, when ultrasound is alternately transmitted toward and received from a plurality of sites once each, the pulse repetition frequency (PRF) corresponding to the sampling frequency becomes smaller, depending on the number of range gates (observation points). Therefore, there is a problem in which the Doppler velocity range of a Doppler spectrum image becomes small, resulting in an aliasing phenomenon (folding phenomenon). Thus, the method according to the prior art is not suitable for measuring the circulatory system.
For example, when ultrasound is alternately transmitted toward and received from two sites once each, the pulse repetition frequency PRF becomes half. Therefore, the Doppler velocity range becomes half, and a folding phenomenon occurs. Specifically, if the blood flow velocity is 60 cm/s when the pulse repetition frequency PRF is 4 kHz and the sight depth is 15 cm, the Doppler spectrum image becomes subject to a folding phenomenon.
In addition, conventionally, the blood flow information at each observation point is acquired by setting a plurality of range gates (observation point) on the same scanning line. In this case, because observation points can only be set on the same scanning line, it is difficult to obtain the blood flow information at a plurality of observation points in the cardiac chambers. Therefore, this method is also unsuitable for measuring the circulatory system.